How many acres are there in one square mile?

A square mile is defined as the area of a square with sides that are each one mile in length. Since one mile is equal to 5,280 feet, you can calculate the area in square feet by squaring the length of one side:

1 mile x 1 mile = 5,280 feet x 5,280 feet = 27,878,400 square feet.

To convert square feet to acres, you need to know that one acre is equal to 43,560 square feet. Therefore, to find out how many acres are in a square mile, you can divide the total square feet in a square mile by the number of square feet in an acre:

27,878,400 square feet ÷ 43,560 square feet/acre = 640 acres.

This calculation confirms that one square mile is equivalent to 640 acres, hence making this the correct answer.